Investigation of mild steel corrosion inhibition in acidic media by Viola extract based on bulk and nanometer size

In the present work, the inhibition performance of Viola extract based on bulk and nano size as a green corrosion inhibitor on mild steel in 0.5 M phosphoric acid and 1M hydrochloric acid solutions is investigated using different techniques (potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Optical microscopy). The gained results demonstrated that various concentrations of Viola Extract (bulk and nano) inhibited the corrosion of the alloy in both of the acid solutions. The temperature impact on corrosion rate without/with this extract was examined. Certain thermodynamic parameters were determined based on the temperature impact on inhibition and corrosion processes. The adsorption mechanism of the extract on the alloy was explored using the Langmuir adsorption isotherm. A mixed mode of adsorption was observed, wherein the nano-sized extract in 1.0 M HCl predominantly underwent chemisorption, while the bulk-sized extract in 1.0 M HCl and both bulk and nano-sized extracts in 0.5 M H3PO4 were primarily subjected to physisorption. Scanning electron microscopy (SEM) and Optical microscopy analyses were employed to scrutinize alloys’ surface morphology.

In the recent years, many people have become interested in green chemistry.the principle of "green chemistry" refers to efforts toward establishing a comprehensive approach to chemical risk management.This concept is based on the ideas of sustainability, reducing environmental consequences, and preserving natural resources for the following centuries 17 .Researchers are also finding ways to make less waste and avoid dangerous substances in the products.They are paying a lot of attention to green inhibitors because they can help prevent corrosion and can be renewed, are environmentally friendly, can be broken down by nature, and are safe 18 .So, Scientists are trying to find green ways to stop corrosion in effective but less harmful ways 19 .
In recent times, researchers have turned their attention to plant extracts as a means to develop alternative, cost-effective, and environmentally friendly corrosion inhibitors [20][21][22][23][24][25][26] .Arthur et al. 27 surveyed the inhibitory impact of Acalypha chamaedrifolia leaf extract on mild steel corrosion in hydrochloric acid.Zaher et al. 28 examined Ammi visnaga L. extract as a corrosion inhibitor for mild steel in HCl solution, achieving an inhibition efficiency of 84%.Shahmoradi et al. 29 demonstrated the efficacy of quince seed extract as a mild steel corrosion inhibitor in 1 M HCl solution, with a corrosion efficiency of 95% at an extract concentration of 800 ppm.Theoretical and empirical surveys identified Inula viscosa extract as an efficient inhibitor for mild steel corrosion in 1 M HCl, providing up to 92% efficiency 30 .Eucalyptus plant leaf extract proved to be a proficient natural corrosion inhibitor in 0.5 M H3PO4 and 0.5 M H 2 SO 4 solutions for mild steel 31 .Aqueous black mustard seeds were investigated as a sustainable green inhibitor in 2 M H 2 SO 4 for mild steel corrosion 32 .Boudalia et al. 33 explored the corrosion inhibition impact of Artemisia essential oil for carbon steel corrosion control in 1 M H 3 PO 4 solution.Aisha Hussain Al-Moubaraki et al. 34 examined the inhibitory effects of Natural Plant Extracts of Zingiber zerumbet (ZZAE), Fraxinus excelsior (FEAE), and Isatis tinctoria on mild steel corrosion in phosphoric acid.
Nanostructured materials have been studied considerable because of their broad range of prominent applications because nanostructures exhibit novel size-dependent properties, such as chemical properties, that extensively differ from their bulk materials, that exhibit great potential in the novel fields.The increasing trend of nano-scale production particularly for encapsulating herb and spice extract has been reported to have some advantages such as improving bioavailability, biological activity and stability as well as controlling the release of bioactive compounds 35 .
The Viola genus, belonging to the Viola cease family, is one of the largest genera, encompassing 525-600 species distributed across most regions of the world and categorized into 14 sections and numerous sub-sections 36 .Renowned for its medical features, including anthelminthic, antioxidant, anti-inflammatory, analgesic, and antidepressant effects, it has been documented for treating different neurological disorders 37 .This perennial herb typically reaches heights of 8-20 cm, characterized by elongated, slender arrow-shaped leaves that often widen from the base, lacking stems and measuring approximately 2.4 inch (6 cm) in length with a V-shaped sinus base.Different parts of the Viola plant are shown in Fig. 1 37 .The compounds identified in Viola species include rutin, isovitexin, and kaempferol-6-glucoside (Fig. 2) 38 .
Viola Extract is a great option to use as an eco-friendly green inhibitor because it doesn't have any dangerous metals or harmful substances.It also has some advantages such as being easy to get to, the friendly environment and high accessibility.Lately, it has been started applying as green corrosion inhibitors instead of organic corrosion inhibitors.In this study, investigated how green corrosion inhibitors can protect mild steel (st-37) from corrosion.Specifically looked at the inhibitory effect of Viola extract at both in bulk, and nanoscale when exposed to acidic environments.Besides, two methods are utilized, potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS), to investigate the corrosion process.As far as we are aware, this extract both in bulk, and nanoscale, is being investigated for the first time for its corrosion inhibition effect on mild steel.

Materials
Materials were ready to use without any additional cleaning or purifying.These materials were obtained from Arshanzist Youtab Company.To make the liquids needed for the experiment, utilized some chemicals: phosphoric acid, hydrochloric acid, ethyl alcohol, methanol, and distilled water.

Preparation of work electrodes to electrochemical measurements
The work electrodes for the corrosion measurements were prepared of mild steel.The samples had a size of 1 square centimeter for all tests.The abaraded side of the metal sheets were made shiny by using different types of sandpaper (100, 400, 1000, and 2500 grit).

Viola extract preparation
The Viola plant contains eaves, flowers and seeds that were bought from the markets in Iran.To get rid of dust, the plants were washed and then they were left in a cool, shady area inside a room until they were dried.At normal room temperature and without any light, 100 g of dried Viola plants were put in distilled water for 72 h.The extra liquid was removed by heating it in a special container at 40 °C, after removing any solid particles.The leftover material weighed 2. 0 g.

Declaration for the usage of plant materials
We declare that in this research, we did not use or not to use any plants (either cultivated or wild) irrespective of any location.Experimental research and field study in this study has complied with the IUCN Policy Statement on Research Involving Species at Risk of Extinction.The use of plants in the present study complies with international, national and/or institutional guidelines.

Preparation of Nanosized Viola extract
Viola nano micelle was produced using heated absolute ethanol and isopropanol 400 as solvent, Tween 80 and sodium caseinate dissolved in distilled water as an emulsifier by dissolution method.Firstly, the solvents (absolute ethanol and isopropanol 400) were heated to 40-42 °C.0.08 g of the emulsifiers (Tween 80 and sodium caseinate 1:1) and the specified amount of Viola extract (2 g) were dissolved in deionized water and heated to reach the solvent's temperature.The solvent phase was added slowly to the aqueous phase containing Viola and mixed for 50 s by a homogenizer at 12,800 rpm to create Viola nanomicelles.The produced sample was stored in a closed container in the refrigerator for the next steps.

Solutions preparation
The electrolytes were made by mixing distilled water with concentrated chemicals.The first liquid was prepared by mixing analytical grade H 3 PO 4 from Merck with distilled water to make a 0. 5 M H 3 PO 4 solution.The second liquid was made by mixing concentrated HCl with distilled water to make a 1. 0 M HCl solution.Before each exploration, the test solution was arranged by blending the Viola extract with the corrosive medium.Two experiments were conducted to make sure the results could be repeated.The concentrations of the extract in the solutions were 100, 150, 200, and 250 ppm for 0. 5 M H 3 PO 4 solution and 1. 0 M HCl solution, the concentrations were 75, 100, 125, and 150 ppm.

Electrochemical evaluations
The reference, counter, and working electrodes were provided by three-electrode cells that contained Ag/AgCl, Pt electrodes and mild steel, respectively.The Tafel polarization curves were plotted by setting the rate of the polarization scan at 1 mV/s.The potentiodynamic polarization and EIS tests were conducted using the potential and frequency ranges of − 200 to + 200 mV versus Ag/AgCl and 100 mHz to 100 kHz, respectively.The potential stability over 30 min before each polarization and impedance test under temperature conditions of 25 ± 1 °C.Finally, NOVA 1.11 software with URL link https:// www.advan cedun insta ller.com/ Nova-1_ 11-7fa4b 72b3e a3964 ce9fb ae4e0 a289e f6-appli cation.htm was employed to analyze the obtained curves.

Temperature impact
The potentiodynamic polarization technique was utilized to study the temperature impact on the mild steel corrosion rate in 0.5 M H 3 PO 4 and 1 M HCl when different Viola extract concentrations were absent and present on bulk while also examining the nano-size at a temperature range of 25-45 ± 1 °C.

Examination of surface morphology
The working electrode's surface morphology was examined by optical images after alloy plunging over 24-h in both solutions under room temperature when the optimum extract concentrations were absent and present with bulk and nanometer size.

SEM, DLS analysis
The histogram of the SBL nanosizer of NPs in Fig. 3 shows that the mean diameter of particle size is located in nanometer range but not really under 100 nm.The reported results showed that NPs had a narrow size distribution and a homogenous dispersity.The morphology of nanosized extract was observed by SEM.Different magnifications of the images are shown in Fig. 4. The spherical particles have grown and some agglomeration was detected.

Potentiodynamic polarization
The inhibition concentration impact on the alloy corrosion was examined through Potentiodynamic polarization measurements, considering 0.5 M H 3 PO 4 and 1.0 M HCl conditions when different Viola extracts (bulk, and nanosize) concentrations were absent and present.As shown by the experimental results, there was a significant decrease in the corrosion current density as the inhibitor concentration increased up to 150 ppm in 1.0 M HCl, and up to 200 ppm, for 0.5 M H3PO4 including bulk, and nano size of the extract.The resulting plots for acidic conditions are illustrated in Fig. 5.The electrochemical parameters of corrosion potential (E corr ), Corrosion current density (j corr ), cathodic Tafel slope (βc), Anodic Tafel slope (βa), surface coating (θ), inhibitory percentage (IE %) resulting from the polarization investigations are presented in Tables 1 and 2.
The equation below calculates the inhibitory efficiency 39 : In which, j corr and j inh indicate the current density without and with inhibitor, respectively, obtained by extrapolation of polarization plots Tefal lines.As shown, there are a decrease in the corrosion current density (from 847 to 216 μA/cm 2 ) for the alloy in blank HCl solution and solution containing bulk inhibitor and the decrease in the corrosion current density (from 847 to 34 μA/cm 2 ) for the alloy in blank HCl solution and solution containing nanosize inhibitor.There are increase in IE% to 75%, and 96% for HCl solutions with bulk, and nano size of the extract.Also, there are decrease in the corrosion current density (from 424 to 28 μA/cm 2 ) for the alloy in blank H 3 PO 4 solution and solution containing bulk inhibitor and the decrease in the corrosion current density (from 424 to 25 μA/cm 2 ) for the alloy in blank H 3 PO 4 solution and solution containing nanosize inhibitor.There are increase in IE% to 93%, and 94% for H 3 PO 4 solutions with bulk, and nano size of the extract.Thus, the bulk and nanosize extract has an inhibitory impact, forming a protective layer when absorbed on the metal surface.Given that higher concentrations lead to higher inhibitory efficiency; it is possible to obtain this protective layer at higher considerations 40 .
The corrosion potential does not typically undergo significant changes by the mixed inhibitors.Based on the experimental results, there are no substantial changes in the corrosion potential at higher inhibitor concentrations under two-acid conditions, highlighting the inhibitor's performance as a mixed type 41 .Moreover, variances in the βa and βc values in comparison with blank solutions indicate the protective effects of inhibitors against the (1) IE% = j corr − j inh j corr × 100

Electrochemical impedance spectroscopy
Viola extract various inhibitory concentrations on mild steel were considered to perform EIS tests in both acidic media and examine the adsorption mechanism, whose resulting Nyquist plots are shown in Figs. 6 and 7.The inhibitor addition to hydrochloric and phosphoric acid in respective concentrations of 150 ppm and 200ppm increased the resulting Nyquist plots' diameter, subsequently increasing the charge transfer resistance (R ct ).
The impedance difference and low frequencies led to different R ct values.The following equation is utilized to calculate the inhibition efficiency 43 :    The plot simulation and evaluation concerning the equivalent circuit represented in Fig. 8 were conducted for more accurate electrochemical impedance plot analysis.R s and R ct show the solution and charge transfer resistances, respectively.The below formula is used to express the constant phase element (CPE) impedance 44 : In which, A, ω, and n represent the proportion coefficient, the angular frequency, and the coefficient of surface roughness, respectively, and i 2 = − 1. CPE will have a similar function to a pure capacitor in the case of n = 1.
The below formula is used to obtain double-layer capacitance 45 : (2)  www.nature.com/scientificreports/C dl and ω max are the respective symbols for the double layer capacitance and the maximum frequency resulting from the Nyquist plot for each analysis.
Tables 3 and 4 display the properties of the alloy when treated with various concentrations of Viola extract (in both bulk and nanosize forms) in the acidic media.This includes measures for instance the accuracy of the data (chi-square), the capacity of the protective layer on the surface (C dl ), charge transfer resistance (R ct ), and the inhibition efficiency (IEI%).
The half-circles also demonstrate that the percentage of IEI% increases when there is more inhibitor.Researchers have found that smaller particles have a higher IEI% compared to larger particles of the same substance in both acid solutions.
Furthermore, when there is more Viola, R ct increases.This is because of more extract adsorption on the steel surface, which helps protect against the corrosive media 46 .Additionally, the inhibitor provides better shielding against ions causing corrosion.When there is a lot of inhibitor in the solution (up to 150 ppm for 1. 0 M HCl and up to 200 ppm for 0. 5 M H 3 PO 4 ), the values for R ct and IEI% are the highest, reaching 80%, 96%, 91%, and 93% respectively.This increase demonstrates that the inhibitor creates a protective layer on the surface of the alloy, which helps stop corrosion.The rate of a chemical reaction decreases when added more Viola extract because it removes the inhibitor from the surface of the metal.As the concentration of the extract increased, the electric double-layer capacitor, became weaker.This might be because the constant for the electric double layer decreased 47 .In this situation, the inhibitor molecules stuck to the surface and took place with the water molecules that were originally there.The C dl decreased because the inhibitor concentration increased because the inhibitor molecules had a weaker electrical field compared to water molecules, causing them to be less organized in the layer between the two materials 48 .Researchers found that the extract can create a protective layer on steel to stop corrosion, showing that Viola extract is effective in preventing corrosion on the alloy.

Adsorption isotherm
Various factors, including the type of material, surface charges, corrosive conditions, ambient pH, concentration of inhibitor, distribution of charges on the inhibitor, and functional groups on the inhibitory molecule, all affect www.nature.com/scientificreports/ the inhibitors' adsorption mechanism 49 .There are typically two adsorption types, one of which (physical) needs a charged metal area and charged species in the solution.The second type (chemical) is characterized by electron sharing or transfer between the inhibitor species, requiring an inhibitor with a lone pair or a free electron and an empty orbital metal 50 .One of the main applications of adsorption isotherm in solid-liquid systems is investigating the inhibitor impacts.Here, adsorption compounds are related to the inhibitor on the surface and the soluble mass.The inhibition strength can be examined on the alloy surface.Various isotherms, including Langmuir, Temkin, and Freundlich, were reviewed to evaluate the adsorption isotherm in both acidic media.This indicates indicating the best confirmation by Langmuir adsorption isotherm for the inhibitor in both media.The equation below is used to draw Langmuir adsorption isotherm plots 51 : which, C, K ads , and θ are the inhibitory concentration, the adsorption equilibrium constant, and the surface coating resulting from the formula below utilizing the inhibitory efficiency, as the potentiodynamic polarization plot outcome: Hence, considering C/θ plot in C (Eq. 5), a straight line can be achieved, depicting the inhibitor obedience from Langmuir adsorption isotherm in such acidic conditions (Fig. 9).

Impact of temperature
The extract's optimal concentration was considered to examine the temperature impact on various parameters, including corrosion current, corrosion potential, surface coating, and inhibitory percentage in the range of 25-45 °C.Figures 10 and 11 highlight the polarization plots, while Tables 5 and 6 summarize the parameters associated with the evaluation of the temperature impact.
Given various processes resulting from the increasing temperature, its effects on the metal-acid inhibition are significantly complicated, including instances such as the inhibitor adsorption on the metal surface or decomposition under increased temperatures.The oxidation rate and reduction reactions on the metal surface are steads up under rising temperatures, preventing the uniform film formation by the inhibitor and resulting in the corrosive ions' access to the alloy surface, subsequently increasing the rate of corrosion 50 .An interesting point is the physical adsorption of inhibitory molecules on the metal surface due to the inhibition reduction at higher temperatures.On the other hand, chemical adsorption would be characterized by an opposite behaviour at higher temperatures.Thus, these two media represent a primarily physical adsorption on the alloy surface since higher temperatures have decreased the inhibition of the alloy in hydrochloric and phosphoric acid 50 .
In general, the decline in inhibition efficiency as temperature rises can be due to the diminishing time gap between desorption and adsorption of inhibitor molecules on the metal surface 52 .Consequently, the metal surface is subjected to the acidic medium for a shorter duration at higher temperatures, leading to an accelerated corrosion rate.Consequently, the percentage of inhibition efficiency declines at elevated temperatures.
For the assessment of the durability of the examined inhibitors across the experimental temperature range, the ΔIE (%) values for 150 ppm bulk and nano-sized extract in 1.0 M HCl, and 200 ppm bulk and nano-sized extract in 0.5 M H 3 PO 4 are computed as outlined below 53 .
(5)    7. The nano size of the extract in 1.0 M HCl exhibited greater stability with increasing temperature, and under these conditions, the extract's nano size was the most effective inhibitor in 1.0 M HCl.The inhibition efficiency of the nano size of the extract in 1.0 M HCl demonstrated minimal change with temperature rise, suggesting a stronger adsorption bond of the nano size of the extract in 1.0 M HCl on the surface.It proves the chemisorption of the extract's nano size in 1.0 M HCl.Hence, it is inferred that both physisorption and chemisorption are involved.Conversely, the inhibition efficiency of the bulk size of the extract in 1.0 M HCl and both sizes of the extract (bulk and nano) in 0.5 M H 3 PO 4 exhibited a significant change with increasing temperature, indicating primarily physical adsorption.
The corrosion rate dependence on the temperature is shown by the Arrhenius relation 54 : The corrosion current density, the frequency factor, the metal dissolution reaction's activation energy, the gas constant (8.314J K −1 mol −1 ), and the absolute temperature (K) are indicated by j corr , A, E a , R, and T, respectively.As shown in Fig. 12, the slope of the Ln j corr plot versus 1/T was used to measure the activation energy.Table 8 indicates the inhibitor's estimated activation energies, highlighting an increase in the activation energy of the mild steel's corrosion reaction by the inhibitor addition to the solutions.

Thermodynamic parameters
When the Langmuir adsorption isotherms are plotted for the inhibitor in the experimental solutions, a graph intercept of 1/K ads is obtained, in which the adsorption equilibrium's constant is represented by K ads.
Calculations of the free adsorption energy values followed the K ads measurement utilizing the following equation: Table 4 indicates the ∆G˚( ads) values for the inhibitor in both conditions, Besides, ∆H˚a ds is determined using the equation below 55 :  In which, T, A, C, R, ∆H o ads , and θ represent the absolute temperature in kelvins, constant frequency, the inhibitor concentration, the gas constant, the adsorption heat, and the surface coating created by the inhibitor molecules, respectively.
Figure 13 shows the plotting of Ln   In conclusion, in comparison to findings from other researchers, it is inferred that Viola extract exhibits the smallest optimal concentration yet maintains effective performance.Thus, utilizing Viola extract in nanometer size allows for a substantial reduction in the required inhibitor concentration while enhancing corrosion resistance and efficiency.This presents an economical, environmentally friendly, and effective approach to mitigating mild steel corrosion in an acidic medium.Therefore, Viola extract emerges as a promising candidate for enhancing the corrosion resistance of mild steel alloy in 0.5 M H 3 PO 4 and 1.0 M HCl solutions.

Figure 3 .
Figure 3.The mean size of produced nanoparticles recorded by the nanosizer equipment (DLS technique).

Figure 4 .
Figure 4.The images of nanosized extract using Scanning electron microscopy (SEM).

Figure 5 .
Figure 5. Polarization curves for st-37 in 1.0 M HCl (a) based on bulk, and (b) nano size of Viola, and in 0.5 M H 3 PO 4 (c) based on bulk, and (d) nano size of Viola at 25 ± 1 °C.

Figure 6 .
Figure 6.The (a, b) Nyquist plots for st-37 with different concentrations of Viola based on bulk, and nano size, in 1M HCl.

Figure 7 .
Figure 7.The (a, b) Nyquist plots for st-37 with different concentrations of Viola based on bulk, and nano size, in 0.5 M H 3 PO 4 .

Figure 8 .
Figure 8. Equivalent circuit to estimate impedance diagrams.

Figure 11 .
Figure 11.Effect of temperature on the polarization curves in 0.5 M phosphoric acid solution (a) without inhibitor (b) with 200 ppm of inhibitor (c) with 200 ppm of inhibitor (nano).

Figure 12 .
Figure 12.Arrhenius slopes calculated from corrosion current density for mild steel in (a) 1.0 M HCl solutions, and (b) 0.5 M H 3 PO 4 at 25 ± 1 °C.

Figure 13 .
Figure 13.Plots of Ln (θ/1-θ) versus 1/T for mild steel in (a) hydrochloric acid solution containing 150 ppm of inhibitor and (b) phosphoric acid solution containing 200ppm of inhibitor, at different temperatures.

Figure 14 .Figure 15 .
Figure 14.The images of the st-37 surface after 24 h immersion in 1.0 M HCl solution in the (a, b) absence, (c, d) presence of 150 ppm of Viola extract in bulk size (e, f), and presence of 150 ppm of Viola extract in nano size using optical, and Scanning electron microscopy, respectively.

Table 1 .
Corrosion parameters derived from polarization curves for st-37 in HCl solution with (a) inhibitor, (b) nano inhibitor.

Table 2 .
Corrosion parameters derived from polarization curves for st-37 in H 3 PO 4 solution with (a) inhibitor, (b) nano inhibitor.C/

Table 3 .
Corrosion parameters derived from Nyquist curves for st-37 in HCl solution in the absence, and presence of different concentrations of inhibitor based on bulk, and nano size, at 25 ± 1 °C.

Table 4 .
Corrosion parameters derived from Nyquist curves for st-37 in H 3 PO 4solution in the absence, and presence of different concentrations of inhibitor based on bulk, and nano size, at 25 ± 1 °C.C/

Table 5 .
Corrosion parameters obtained from polarization measurements in 1 M HCl (a) without inhibitor (b) with 150 ppm of inhibitor c) with 150 ppm of inhibitor (nano) at different temperatures.

corr /μA cm −2 − E corr /mV IE%
The ΔIE (%) value indicates the general reduction in inhibition efficiency with raising the temperature from 25 to 45 °C.Smaller ΔIE (%) values suggest greater stability of inhibitor with rising temperature, leading to stronger inhibitor adsorption on the surface.The calculated ΔIE (%) values are presented in Table

Table 6 .
Corrosion parameters obtained from polarization measurements in 0.5 M H 3 PO 4 (a) without inhibitor (b) with 200 ppm of inhibitor (c) with 200 ppm of inhibitor (nano) at different temperatures.

Table 7 .
ΔIE(25-45)% values for 150 ppm bulk and nano size of the extract in 1.0 M HCl and 200 ppm bulk and nano size of the extract in 0.5 M H 3 PO 4 .

Table 8 .
Kinetic and Thermodynamic parameters for adsorption of inhibitor in (a) 1.0 M HCl, and (b) 0.5 M H 3 PO 4 solutions at 25 ± 1 °C.
E a (